1. Field of the Invention
The present invention relates to a ball grinding machine. More specifically, the invention relates to a ball grinding machine for machining and manufacturing ceramic bearing balls, which employs a non-rotating upper plate and a rotating lower plate for grinding ceramic balls in between and for achieving improved ball sphericity, surface roughness, and uniformity.
2. Description of Related Art
Precision ceramic bearing balls are considered indispensable mechanical parts today and will play an even more important role in the foreseeable future, especially in fields where they are not easily replaceable by conventional steel bearing balls, for example aerospace and precision mechanics. Precision ceramic bearing balls possess such advantages as being able to function under high operating temperatures and speed without apparent wear and tear, and they can also function reasonably well under extremely corrosive environment. The characteristics of precision ceramic bearing balls make them a key component in the fabrication of high-speed precision bearing tools. However, since a typical ceramic workpiece has a very hard and brittle surface, it can not be as easily and efficiently machined by a conventional ball grinding or lapping machine as a typical steel workpiece. The ball grinding machine according to the present invention, therefore, solves the above-mentioned problems by providing a simple but practical mechanism for easy maintenance, high production rate, and uniform machining quality.
The machining steps for mass-producing ceramic bearing balls typically involves putting ceramic material of crude spherical shape into different ceramic ball grinding machines after they are sorted or graded by the degree of sphericity and surface roughness. Thus, a rough ceramic ball can proceed from a rough grinding machine to finer grinding machines then finally to a lapping machine for forming precision ceramic bearing balls. The difference between the production steps of a steel bearing ball and a ceramic bearing ball lies in the preparation of the steel and ceramic materials in their crude shapes. The crude material for the production of steel bearing balls is typically prepared by shearing a steel rod into cylindrical blanks then shaping each cylindrical blank into a crude spherical shape by a pressing machine fitted with a pressing mold. On the other hand, the raw material for the production of ceramic bearing balls typically starts with pressurizing a predetermined amount of ceramic powder with a pressing mold into a spherical shape and followed by a sintering step for forming a rough ceramic ball. There are different pressing methods available for pressurizing the ceramic powder into a rough ceramic ball, including mechanical pressing, cold isostatic pressing, and hot isostatic pressing.
Furthermore, each bearing ball is graded by degree of sphericity and surface roughness, wherein the most important factor contributing to the grade of a bearing ball is the design of the ball grinding mechanism, which includes such variables as grinding rate, grinding load, and material compositions of the bearing ball, grinding wheel, and slurry. In general, the ball grinding mechanism directly affects the patterns of grinding movements, the damping effect, and thus the result of a grinding finish. Up to the present, the most commonly applied ball grinding mechanism or method for grinding a steel bearing ball is the grinding wheel method, which provides satisfying ball sphericity and surface roughness. Whether the grinding finish of a steel bearing ball is to be fine or rough is basically determined by the size and the roughness of the abrasive grains of the grinding wheel. Traditional grinding wheel method performs steel bearing ball grinding by batch loads, but since the rotation rate of the grinding wheel is typically below 100 rpm, it therefore has a low production rate. Also due to the fact that the grinding load or the pressure exerted by the one-piece grinding wheel towards each of the steel bearing balls is the same, it is difficult to quickly adjust the grinding load or pressure by the requirement of each batch load.
In recent years, emphasis has been placed on how to actively adjust the grinding rate and the grinding load to raise the production rate and grinding quality of the bearing balls, whereas the grinding load is the pressure exerted by the grinding wheel to a steel ball through three contacting surfaces. Grinding methods with such feature mainly include the magnetic fluid grinding method, the controlled rotating shaft grinding method, the planetary gear type grinding method, and the magnetic levitation grinding method, etc. However, most of the prior ball grinding methods incorporates a single grinding track design, so production by batch loads can not be carried out conveniently.
Furthermore, the grinding mechanism of a ball grinding machine dictates the limit of grinding quality by which a bearing ball can be machined. That is, the speed variation of a bearing ball spinning and orbiting in the grinding track depends on the revolving speeds of the bearing ball relative to the above-mentioned three contacting surfaces. In addition, the speed variation can be further attributed to the friction coefficients inherent to the bearing ball and the three contacting surfaces. By manipulating the combined speed and directional variations of the bearing ball spinning and orbiting in the grinding track, the surface of the bearing ball can be evenly and uniformly ground.
In 1976, after trying fourteen fundamentally different ball grinding machines on the same type of bearing balls, it was determined by Inagaki and Abe, that the grinding wheel method is the most effective grinding method to achieve the best ball sphericity and surface roughness, and is also by far the most productive method. FIG. 1 shows an example of such grinding wheel type ball grinding mechanism.
The ball grinding mechanism shown in FIG. 1 is comprised of a rotating grinding wheel 1 (the lower plate) and a fixed circular guiding plate 2 (the upper plate) disposed such that both of their working surfaces are positioned in parallel to each other with just enough separation in between for steel bearing balls 3 to be ground beneath the fixed circular guiding plate 2 and by the grinding surface of the concentric V-type annular grooves (not shown) of the lower plate. The two opposite surfaces in the V-type annular groove and the bottom surface of the upper plate together constitute the three contacting surfaces between each of the steel bearing balls 3 and the grinding mechanism. As the lower grinding wheel 1 rotates, the steel bearing balls 3 can be ground or machined accordingly. If the production of the steel bearing balls 3 is to be performed in batch mode, where thousands of steel bearing balls 3 are to be ground at a time, a fan-shaped guiding chute 4 is formed on the edge of the lower plate towards the direction of the center point in order to grind the steel bearing balls 3 in a continuous fashion. Efforts have also been made to improve the continuous feeding mechanism for the bearing balls such that the upper and lower circular plates of a grinding mechanism and drive shaft are all positioned at an inclined angle as disclosed in Taiwanese Patent No. 272156, 1996. Another design that features this improved continuous feeding mechanism has a horizontal positioning of the plates, such as the one disclosed in U.S. Pat. No. 5,301,470, April 1994, by Sato. On the other hand, in order to improve the feeding mechanism for the grinding fluid, or slurry, a plurality of slurry-supplying passages are formed in a fixed, non-rotating plate (not shown), such as the mechanism disclosed in Japanese Patent No. 7-314325, 1995.
Later in 1988, Umehara and Kato published a paper on the subject of magnetic fluid grinding method for machining precision ceramic balls 10. U.S. Pat. No. 4,821,466 was later issued to Kato et al. in April 1989 based on the same apparatus and method. Umehara et al., on the other hand, acquired a Japanese patent, NO. 8-257897, in October 1996 on a related mechanism design. As shown in FIG. 2, such method uses a slurry having abrasive grains suspended in a magnetic fluid as the grinding agent, referred to as magnetic grinding fluid 5 hereafter, wherein a non-magnetic material is formed into a lower floating plate, or a float 6, and positioned within the restriction of a guide ring 7 filled with the magnetic grinding fluid 5. In addition, a levitation force generated magnet sets 8 was adopted to act as the grinding load, and a cylindrical rotating shaft 9 with a chamfered end performs the function of an upper plate in driving the precision ceramic balls 10 into high-speed rotation. The aforementioned grinding method proclaims the advantages of high machining rate and fine ball sphericity and surface roughness.
A magnetic fluid with grinding wheel method was disclosed by Chang and Nakajima in 1997, utilizing the grinding mechanism as depicted by FIG. 3. This mechanism improves the prior magnetic fluid grinding mechanism by adding an upper diamond grinding wheel 11 onto the rotating shaft 9 of FIG. 2 and by adding a gum diaphragm 12 in a lower position and a taper thrust 13 on top of a non-magnetic float 6. A magnetic fluid 14 is filled below the gum diaphragm 12 and above a combination of magnet sets 8 to exert a grinding load, and a grinding fluid 15, is filled above the gum diaphragm 12 to assist grinding by providing abrasive grains in suspension. Furthermore, FIG. 4 shows that compression springs 16 are used in place of the magnet sets 8 shown in FIG. 2, in a non-magnetic fluid grinding method disclosed by Chang and Childs.
In 1990, a controlled rotating shaft grinding method for manufacturing precision ceramic bearing balls was disclosed by Kurobe et al. Referring to FIG. 5, the method includes controlling and varying the inclination angle of the spin axis of each ceramic bearing ball, wherein the inclination angle is the angle of spin axis with respect to a horizontal plane. As shown in FIG. 5, a grinding mechanism according to such method comprises a first motor 17, a second motor 18 and a rotary joint 19 controlling the rotating speeds of an upper lapping plate 21, a lower inner lapping plate 22, and a lower outer lapping plate 23, each independently driven through a synchronous belt 20 mechanism. Moreover, tiny abrasive diamond grains are suspended in a water-based fluid as a slurry, and an grinding load exerted upon each of the ceramic bearing balls is actuated through compressed air. In 1991, Ichikawa et al. disclosed a precision lapping machine for ceramic balls as shown in FIG. 6. This particular precision lapping machine resembles a conventional lapping machine, wherein the center of an upper rotating plate 24 of smaller diameter is offset to the center of a lower rotating plate 25 of larger diameter. Furthermore, a first lapping tool 26 and a second lapping tool 27 carrying a first grinding load 28 and a second grinding load 29, respectively, are incorporated into the upper rotating plate 24 for the lapping of precision ceramic balls. A similar design is disclosed by a Japanese Patent No. 7-77705.
Also as shown in FIG. 7, The Institute for Manufacturing Technologies of Toshiba Japan disclosed a magnetic levitation grinding mechanism, wherein the rotation speeds with respect to each of the three contacting surfaces of a bearing ball are controlled and adjusted by three separate motors, which resembles the controlled rotating shaft grinding method depicted by FIG. 5. This particular grinding method is characterized by its use of magnetic levitation force as the grinding load.